1. Technical Field
The present invention relates generally to the control signal write circuits of a video apparatus, and more particularly, to improvements in a leader picking signal write process, in the control signal write circuits (those which will be described, in this specification, include video apparatuses incorporating cameras, stationary video tape recorders and video apparatus incorporating monitors) having a leader picking function.
2. Background Art
With the recent diffusion of video tape recorders, the function of the video tape recorder is increasingly intensified and diversified. A leader picking function is no exception.
The leader picking function is, such that, certain tag information is stored at each start point of recording (i.e., at the point from which recording is started) on a video tape so that the tape is automatically stopped by detecting the information at the time of fast feeding or rewinding. The leader picking function makes it possible to find out the leading end position of a program at the time of fast feeding and rewinding even when a plurality of broadcasting programs have been recorded thereon.
As leader picking systems, there are presently VISS (VHS Index Search System), VASS (VHS Address Search System) and the like.
In the case of the former system, leader positions are stored in the form of tag information and the tape is stopped at the predetermined position each time the information is read, whereas in the case of the latter, leader positions are stored in the form of predetermined address data, and when an address is designated, the position of the data conforming to the designated address is detected so that the tape is automatically stopped. The control signal written onto the control track of a video tape is generally used to read a leader picking signal or an index signal for obtaining leader picking information or data, or a part of an address signal for determining a stopping position of a video tape (hereinafter referred to as "leader picking signal") when such a leader picking function is practically utilized. The leader picking signal is written or erased by changing the duty ratio of the control signal.
As illustrated in FIG. 4, it is assumed that a video tape 10 is caused to travel in direction of arrow Y at the time of playback. A control track A.about.A' is provided on one side of the video tape 10. In this case, the side A represents what is to be read and the side A' what has already been read.
A control signal 10a is stored on the control track A.about.A' and used as a synchronizing signal (timing signal) when a video signal 10c to be stored on the video track are regenerated. In this case, an "ON/OFF" signal having a predetermined duty ratio (i.e., a signal which has been magnetized to the N- and S-polarity on the video tape 10) is normally used for the purpose. With the control signal for use in only specifying a position on the video track, at least the timing position of either a transition point from "ON" to "OFF" or a transition point from "OFF" to "ON" should only be identified. Therefore, there is no problem as long as the transition point is not changed, even when the duty ratio is changed. With this in mind, the leader picking signal is recorded by changing the duty ratio of the control signal.
Of the control signal, the portion used for the synchronizing signal is not directly related to the aforementioned duty ratio and this portion is assumed equivalent to a point where the magnetization is changed from the S- to N-polarity on the video tape 10. This transition point is a point at which the control signal is changed from "OFF" to "ON" when it is read by a control head 7 from the control track, that is, the point is equal in position to the timing of its rise. Consequently, the leader picking signal is written or erased by rewriting the duty ratio of the control signal in such a state that the transition point is rendered unaffected.
Of the control signal, the portion involved in the duty ratio is equivalent to a point where the magnetization is changed from the N- to S-polarity on the video tape 10. Therefore, the leader picking signal to be written or erased is rewritten at this point only. This point at which the control signal is reversely changed from "ON" to "OFF" is equal in position to the timing of the fall of the control signal and has no significance on the part of the control signal itself.
Incidentally, the writing of the control signal, which is dependent on its "ON" or "OFF" state, is based on d.c. recording effected by switching the direction of current from a terminal REC(+) to a terminal REC(-) in the control head or from REC(-) to REC(+) therein. While the control signal is being written, the direction of current is switched at predetermined intervals, whereby the direction of magnetization (i.e., N- and S-polarity) in the magnetic material on the control track A.about.A' is reversed at predetermined intervals.
In the case of the video tape loaded with images in a normal way, the control signal is in such a state that no leader picking signal has been recorded thereon, that is, a normal control signal whose duty ratio is about 60% as shown by 10a is employed and this signal is written onto the control track A.about.A'. On the other hand, a control signal whose duty ratio is about 27.5% as shown by 10b is written onto the control track A.about.A' at a leader picking position where the leader picking signal has been recorded.
When the leader picking signal 10b is written, current for use in writing N-polarity is supplied to the coil of the control head 7 after it has passed the point at which S-polarity of the control signal corresponding to the write position is transferred to its N-polarity (after about 8% in terms of the duty ratio). The direction of current is then changed so that S-polarity is written from a position at a duty ratio of 27.5% and the current is supplied to the coil of the control head 7. Subsequently, control is exercised in such a way that the current flowing through the coil of the control head 7 is suspended before the magnetization is transferred from S- to N-polarity (i.e., before about 10% in terms of the duty ratio).
In order to erase the leader picking signal 10b in such a state that the leader picking signal is read to cause the tape to stop at the position designated thereby, current for use in writing the N-polarity from the rising point of the control signal including the leader picking signal is made to flow through the coil of the control head 7 and the current is switched in such a way that the S-polarity is written from a position at which a duty ratio comes to 60%. This process is actually performed by letting the current for writing the N-polarity flow through the control head 7 after the control head 7 passes the transition point at which the S-polarity is switched to the N-polarity before the position of the leader picking signal 10b (after about 8% in terms of the duty ratio) and letting the current flow through the coil of the control head 7 in the opposite direction from a position where the duty ratio comes to 60% in order to write the S-polarity. Further control is exercised so that the current supplied to the coil of the control head 7 is stopped before the next transition point from the S- to N-polarity is reached (i.e., before about 10% in terms of the duty ratio).
In this way, only the position of the fall portion (the transition point from the N- to S-polarity) of the control signal is changed without changing its rise-up timing (the transition point from the S- to N-polarity) be rewriting to superimposing the control signal. The leader picking signal can thus be written or erased. In the case of the rewriting, the video signal and the like are not rewritten.
The control signal is rewritten as stated above so that the leader picking process is implemented and a part of the control signal is then rewritten. For this reason, there may arise a difference in the degree of magnetization between the control signal already written to the video tape 10 and what is rewritten thereto. Especially when it is attempted to rewrite the control signal so as to write a leader picking signal onto the control signal written by another video tape recorder, there inevitably arises a subtle difference in the degree and position of magnetization.
When such a difference in the degree of magnetization of the control signal ensues at the time of starting of terminating the rewriting, the magnetic flux density varies in that portion different in level when the control signal is read via the control head 7 at the time of playback and the waveform of the signal that has been read accordingly becomes discontinuous. A detection circuit for detecting the rise and fall of the control signal mistakes a transition point where the magnetization ratio varies for leader picking that is, the place where the aforementioned rewriting is started or terminated for the rise or fall timing of the control signal and so malfunctions as to generate a detection signal. The problem is that the image signal may fail to synchronize with the control signal.
In order to solve such a problem, the current flowing through the coil of the control head 7 is gradually increased or decreased when the rewriting of the control signal is started or terminated. As shown in FIG. 4, this process is accomplished by providing a trapezoidal wave generating circuit 11 mainly comprising a capacitor C1 and supplying its output to a drive circuit 12 so as to control current to be passed through the coil of the control head 7 by the operation of a charge/discharge circuit comprising resistors R1, R2 and the capacitor C1. In this way, the rise and fall conditions of the signal for rewriting the control signal to which the leader picking signal is written are made dull (see(L-shaped waveform) of FIG. 4).
Since circuits similar to those illustrated in an embodiment of the present invention in FIG. 1, which will be described later, are used for those other than the aforementioned of FIG. 4, the description of them will be omitted except that like reference characters are given in the embodiment.
In the prior art shown in FIG. 4, the trapezoidal wave generating circuit 11 is complete with one capacitor C1 and switch circuits SW1, SW2 are used to switch charging to discharging and vice versa. If the trapezoidal wave L is formed by using even one capacitor, the capacitor will have to be fitted outside when a control signal write circuit in the form of an integrated circuit (IC) is employed. Moreover, the drooping of the trapezoidal wave L remains irregular as the source voltage and the capacity of a capacitor for use vary. On the other hand, an increase or decrease in current at the time of starting or terminating writing in the case of rewriting has to be caused during the predetermined allowable time. Consequently, the trapezoidal wave L needs a relatively sharp drooping characteristic so that the variation of the write current is completed within a predetermined short period of time. Therefore, the maximum value of the drooping at the time of charging/discharging can not be too small. As a result, the problem of the malfunction of the circuit for detecting the control signal has not satisfactorily been solved yet.